Nuclear Physics Stable nuclei, Nomenclature and units Atomic scale is 10-10m, nuclear scale is 10-15m. For convinience we use the unit fermi (fm) = 10-15m. Nuclear sizes range from 1 fm to 7 fm. Particle physics usually happens at an even smaller scale <-15m. A nuclear species or nuclide is denoted by \(^A_ZX_N\). Here, X is the chemical symbol. Z is the atomic number: the number of protons. A is the mass number: the integer nearest to the ratio between the nuclear mass and the fundamental mass unit (1⁄12 th of mass of \(^{12}_6C\)).